Integrated anti-skid and hydraulic booster braking control

ABSTRACT

A secondary source of braking for large hydraulically braked vehicles utilizing the available power source ( 38 ) provided by typically available anti-skid and traction control braking systems. The anti-skid braking system is used to control the secondary braiding function in a manner optimal to the operating condition of the vehicle. Unique distinctive operating modes are used when the vehicle is either moving ( 79, 81, 83 ), or stationary ( 57, 59, 73 ). The advantages of the stationary mode are fast time response and high output pressure controlled with a minimum of pump operation and fluid movement. The advantages of the dynamic mode are the ability to modulate brake pressure in response to the driver&#39;s command.

This application is a divisional of application Ser. No. 09/090,683,filed Jun. 4, 1998.

The present invention relates generally to a power assisted brakingsystem for a vehicle and more particularly to methods and apparatus foroptimal control and improved time response for a secondary or back-upbraking system.

BACKGROUND OF THE INVENTION

The time-honored skid avoidance technique of “pumping” a brake pedal tocontrol a skid situation has largely been displaced by anti-lock brakingsystems. Many known anti-lock devices operate by cyclically increasingand decreasing a braking force exerted on the wheels so that a slippingwheel having a tendency to lock is permitted to re-accelerate back to aspoed corresponding to the speed of the vehicle. This is typicallyachieved by control valves alternately allowing fluid to flow out of andthen into a brake cylinder to first lower and then raise the brakepressure in the brake system.

Typically anti-lock or anti-skid braking systems utilize a so-calledpump-back scheme or a replenishing scheme during a reapply or buildoperational sequence to maintain a desired level of hydraulic fluid in abrake system. In a pump-back scheme, the same hydraulic fluid isre-supplied from a local accumulator to the brake pad actuators while ina replenish scheme hydraulic fluid comes from a separate source such aseither a hydraulic accumulator or a separate pump and motor.

Most of such anti-lock braking systems are further capable of operatingin a traction control function. A traction control function isestablished by detecting conditions where the rotational speed of afirst powered wheel substantially exceeds that of a second poweredwheel. To provide a power balance in the operation of a vehicle, abraking force is applied to the powered wheel rotating at a higher speedto effectively transferring driving torque back to that wheel withbetter traction. Many anti-lock systems having such a traction controlfeature employ a motor and hydraulic pump or pumps which operateindependent of the service braking system. Such a braiding system isdisclosed in U.S. Pat. No. 5,709,438 wherein a traction control motorand hydraulic pump are called into action to provide a back-up powerassisted braking feature in the event of a malfunction of the primarybraking system. Such malfunctions may occur simply because the engine isnot operating to drive the primary power braking source, ruptured orfailed brake lines, broken power steering pump drive belts, or for avariety of other reasons. This prior patent provides back-up anti-skidbraking in but a single mode. This prior patent represents the point ofdeparture for the present invention.

It is desirable to provide a braking control system of the typedisclosed in the above mentioned U.S. Patent having a back-up featurewhich modulates pressure to the service brakes in the event of loss ofthe primary brake power source. It is also desirable to provide a“wake-up” mode of operation which allows secondary or back-up powerassisted braking with the ignition off.

SUMMARY OF THE INVENTION

The present invention provides solutions to the above problems byproviding a braking control which uses brake pedal input and vehiclewheel speeds or hydraulic pressure available in a primary braking systemto activate a secondary or back-up braking in either a static or dynamiccontrol mode.

In general, the invention provides a power assisted braking system for avehicle having anti-skid capability with primary and secondarypressurized fluid sources and includes an operator actuable pedal fordeveloping an input force to command the application of braking forcehydraulic pressure as well as one or more pressure transducers formeasuring the actual hydraulic pressure applied during braking. Thevehicle velocity is determined and the vehicle deceleration computed.The primary brake system is operable in a normal mode utilizing aprimary source of hydraulic fluid pressure to provide normal powerassisted braking. There is an arrangement for detecting a malfunction inthe primary braking system fluid source which is effective to substitutethe secondary fluid source for the primary fluid source to providecontinued back-up power assisted braking. The system is operable in afirst back-up mode upon detecting a malfunction of the primary brakingsystem, a command for braking, and a first vehicle speed input toprovide anti-skid braking wherein pedal input force is compared withmeasured vehicle deceleration to generate braking pressure build anddecay commands for operating the brake system. The system is operable ina second back-up mode upon detecting a malfunction of the primarybraking system, a demand for braking, and a second vehicle speed inputto provide anti-skid braking wherein pedal input force is compared withcalculated vehicle deceleration to generate braking pressure build anddecay commands for operating the brake system.

Also in general and in one form of the invention, a power assistedbraking system for a vehicle has a primary power source and anelectrically driven back-up power source for supplying pressurizedhydraulic fluid for braking. The system includes a wake-up circuithaving means for sensing depression of the vehicle brake pedal wherebyinput force is created, and means responsive to the brake pedal sensingmeans indicating depression has occurred for enabling the back-up powersource to provide power assisted braking even when the vehicle ignitionis in the off position. The wake-up circuit may include an arrangementto test for adequate hydraulic pressure either when the brake pedal isdepressed or the ignition is turned to the on position and to enable theback-up power source when the hydraulic pressure is not adequate.

Still further in general and in one form of the invention, back-up powerassisted braking includes a pump operable upon detection of amalfunction of the normal power assisted braking and at least onetransducer for sensing the hydraulic pressure applied to the brakeactuators and providing a signal indicative thereof. During back-uppower assisted braking, if the sensed hydraulic pressure exceeds aprescribed value, the supply of hydraulic fluid to or from the actuatorsis blocked and the back-up pump turned off to maintain the brakingpressure at the actuators near the prescribed value.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic representation of the hydraulic portion of abraking system made according to the principals disclosed in thisinvention;

FIG. 2 is a schematic representation of the control arrangement for thesystem of FIG. 1; and

FIG. 3 is a functional block diagram describing the several modes ofoperation of the system of FIGS. 1 and 2.

Corresponding reference characters indicate corresponding partsthroughout the several views of the drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A anti-lock hydraulic braking system 8 for use in a light truck orsimilar vehicle is shown schematically in FIG. 1. A rear wheel drivewith dual rear tires is illustrated, but the principles apply equally toother configurations. The braking system 8 includes solenoid actuatedanti-skid valves 10 and 16 located between an operator-controlledpressure source or master cylinder 11 and hydraulic brake actuators12,12′ for the front wheel brakes 13,13′ and hydraulic actuators 14,14′for the rear wheels 15,15′. Typically, the pressure source 11 is aconventional master cylinder having two separate circuits, one for thefront vehicle wheel brakes 13,13′ brakes and the other for the rearwheel brakes 15,15′. A suitable pressure source is disclosed incopending application Ser. No. 09/015,166 filed Jan. 29, 1998 and nowU.S. Pat. No. 5,960,629, assigned to the assignee of the presentinvention. The vehicle wheels also have rotational speed sensors (53,55)for providing electrical indications of the angular velocities ofindividual wheels to an anti-skid electronic control unit.

When the driver wishes to slow the vehicle, the pedal 23 is depressed todevelop an input force which is applied to piston in master cylinder 11.Movement of the pistons in master cylinder 11 creates a hydraulic fluidpressure which is transmitted from the master cylinder 11 by way ofconduits (brake lines) 25 and 27 to the respective rear and front pairsof brake actuators by way of four individual solenoid actuated anti-skidvalves 16, 17, 18 and 19. The individual wheel anti-skid valves such as16 are normally open to selectively supply braking fluid pressure fromthe source 11 by way of line 27 to the individual brake actuators suchas 12 and 14. Thus, braking fluid flows to the rear wheel brakes 15,15′from the master cylinder 11 via line 27 through valve 31 and line 35 tovalves 16,19 and finally by way of line 37,37′ to the brake actuators14,14′ during normal braking. A similar normal braking fluid flow pathexists for actuators 12,12′ for the front wheels brakes 13,13′. Valvessuch as 10 and 16 function as build and hold valves supplying brakingfluid pressure from either line 27 during normal braking or from theaccumulator 34 during anti-skid or traction control operation.

Under normal operation when the secondary or back-up system is requiredtwo hydraulic valves are actuated. The first valve, such as 32 iscommonly known as an ASV, opens to provide a fluid source path from themaster cylinder 11 to the inlet of the pump 38. The second valve, suchas 31 is commonly known as a USV, closes to permit the build up ofhydraulic pressure by the pumps in the braking circuits and accumulator36. This same technique is applied to both secondary braking andstandard ASR (traction control) modes. In addition it is common for thefront and rear 38 pumps to share a common motor.

The normal mode of braking and anti-skid operation is illustrated by thecontrol circuitry of FIG. 2. The left hand portion of FIG. 2 comprisescomponents of an electronic control unit of the system. The forceapplied on depression of pedal 23 is converted at 39 from an input forceto a target or commanded hydraulic pressure. A pedal switch 24 mayprovide an electrical measure of the degree of pedal input force or thepressure may be otherwise monitored. The actual hydraulic pressure ismonitored at 57 and 59, and these measured values are passed through alow pass noise eliminating filter 45 and then compared at 41 and 43 withthe commanded pressure or desired braking signal. Filter 45 may, forexample, have a cut-off frequency of about 10 Hz. Pressure errorindicative signals for the front and rear braking 14 circuits aresupplied to the PID controller 47. Controller 47 has the transferfunction Kp+Ki/S+Kd*S and functions to combine the current error,previous error and the derivative of the error to generate a pair ofsignals indicative of the required error corrections. The expression forthe transfer function employs conventional notation where division by Sindicates an integration and multiplication by S indicatesdifferentiation. These error corrections are converted at 49 to buildand decay control signals on the output lines such as 61 and 63. Theseoutputs are constant pulse width with the spacing between pulses variedas required. Integrals of the build and decay pulse trains are generatedby control unit 51 which has the transfer function Khu/S. The controlunit 51 controls the solenoids for valves such as 10 and 16.

The outputs of the pressure sensors 57 and 59 may also be monitored tolimit the strain on the pumps such as 38 and several of the valves. Forexample, when the monitored pressure reaches 70 Bar, valves 16 and 19,or 17 and 18 may be shifted to the off position to provide a hold modewhere the corresponding pump may be disabled and the vehicle heldstationary in position, for example, on a hill.

The several modes of operation of the system are best described inconjunction with FIG. 3 which describes the behavior of the systemelectronic control unit. With the ignition switch 65 on and the brakepedal 23 depressed as indicated by pedal switch 24, the hydraulic fluidflow from booster 11 is measured at 67. If the flow is adequate, thebrakes are applied by a signal on line 69. In the event of a pump orengine failure, flow switch 67 provides a low flow signal to enable thewarning lamp and buzzer 71 and to initiate a check of the vehicle speedat 77 from one of the sensors 53 or 55.

If the vehicle is stationary, the system enters the static control mode.In the static control mode, the front and rear pressure sensors 57 and59 are immediately checked for to determine if an adequate pressure, forexample, 70 Bar or above is available for braking. If either is low, thepumps such as 38 are enabled at 73 to build up the pressure and applythe brakes. If both front and rear pressure is adequate, a signal online 75 is sent to actuate the brakes.

If the vehicle is moving, the system enters the dynamic control mode. Ifthe primary power source for power assisted braking is operatingnormally, braking is performed as shown in FIG. 2 with pedal input forcebeing compared to the hydraulic pressure as indicated at 83. If therehas been a malfunction, the vehicle speed is again sensed at 79. If thespeed is above a predetermined value, say greater than 10 MPH, closedloop back-up control is initiated at 81 wherein the pedal input iscompared to the actual vehicle deceleration. In this mode, in essence,the wheel speed as sensed at 53 and 55 is fed back through filter 45 andcompared to the output of 39. If the speed as sensed at 79 is below apredetermined value, say less than 10 MPH, then an open loop back-upcontrol mode is initiated at 83 with the pedal input being compared to acalculated deceleration.

What is claimed is:
 1. A power assisted braking system for a vehiclehaving a primary power source and an electrically driven back-up powersource for supplying pressurized hydraulic fluid for braking, saidsystem including a wake-up circuit having means for sensing depressionof the vehicle brake pedal, and means responsive to the brake pedalsensing means indicating depression of the brake pedal has occurred forenabling the back-up power source to provide power assisted braking evenwhen the vehicle ignition is in the off position.
 2. The braking systemof claim 1 wherein the wake-up circuit includes means to test foradequate hydraulic pressure either when the brake pedal is depressed orthe ignition is turned to the on position and to enable the back-uppower source when the hydraulic pressure is not adequate.
 3. In a powerassisted braking system for a vehicle having anti-skid capability wherecommanded hydraulic braking pressure is compared to vehicle operatingparameters to generate build and decay signals for applying andreleasing the brake actuators thereby effecting a controlled brakingprocess, said process of selecting a vehicle operating parameterappropriate to the current vehicle operating conditions comprising thesteps of: sensing for vehicle motion; determining if a primary brakingsystem is functioning properly by sensing pedal force applied by anoperator to effect a brake application, a flow of hydraulic fluid from aprimary source and a current hydraulic pressure of said hydraulic fluidfrom said primary source; selecting a hydraulic pressure as anoperational parameter if the vehicle is in motion and said primarybraking system is functioning properly, otherwise; sensing the vehiclespeed if the vehicle is sensed to be in motion and said primary brakingsystem is malfunctioning; selecting calculated vehicle deceleration asthe parameter in the event the vehicle speed is sensed to be above aspecified value; selecting a wheel brake pressure as the parameter inthe event the vehicle speed is sensed to be below said specified value;and applying a fixed braking pressure if the vehicle is sensed to not bein motion.
 4. The process of claim 3 wherein the specified vehicle speedis about ten miles per hour.
 5. The process of claim 3 wherein thespecified fixed braking pressure is, about 70 Bar.
 6. In a powerassisted braking system for a vehicle having anti-skid capability wherecommanded hydraulic braking pressure is compared to a vehicle, operatingparameters to generate build and decay signals for applying andreleasing the brake actuators thereby effecting a controlled brakingprocess, said process of selecting a vehicle operating parameterappropriate to the current vehicle operating conditions comprising thesteps of: sensing for vehicle motion; determining if a primary brakingsystem is functioning properly by sensing pedal force applied by anoperator to effect a brake application, a flow of hydraulic fluid from aprimary source and a current hydraulic pressure of said hydraulic fluidfrom said primary source; selecting a hydraulic pressure as anoperational parameter if the vehicle is in motion and said primarybraking system is functioning properly, otherwise; sensing the vehiclespeed if the vehicle is sensed to be in motion and said primary brakingsystem is malfunctioning; selecting wheel brake pressure as theparameter in the event the vehicle is sensed to be in motion; andapplying a fixed braking pressure if the vehicle is sensed to not be inmotion.
 7. The process of claim 6 wherein the specified vehicle speed isabout ten miles per hour.
 8. The process of claim 6 wherein thespecified fixed braking pressure is about 70 Bar.